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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_asio.tutorial.tutdaytime6"></a><a class="link" href="tutdaytime6.html" title="Daytime.6 - An asynchronous UDP daytime server">Daytime.6 - An asynchronous
      UDP daytime server</a>
</h3></div></div></div>
<h5>
<a name="boost_asio.tutorial.tutdaytime6.h0"></a>
        <span class="phrase"><a name="boost_asio.tutorial.tutdaytime6.the_main___function"></a></span><a class="link" href="tutdaytime6.html#boost_asio.tutorial.tutdaytime6.the_main___function">The
        main() function</a>
      </h5>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
  <span class="keyword">try</span>
  <span class="special">{</span>
</pre>
<p>
        Create a server object to accept incoming client requests, and run the <a class="link" href="../reference/io_context.html" title="io_context">io_context</a> object.
      </p>
<pre class="programlisting">    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_context</span> <span class="identifier">io_context</span><span class="special">;</span>
    <span class="identifier">udp_server</span> <span class="identifier">server</span><span class="special">(</span><span class="identifier">io_context</span><span class="special">);</span>
    <span class="identifier">io_context</span><span class="special">.</span><span class="identifier">run</span><span class="special">();</span>
  <span class="special">}</span>
  <span class="keyword">catch</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span><span class="special">&amp;</span> <span class="identifier">e</span><span class="special">)</span>
  <span class="special">{</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cerr</span> <span class="special">&lt;&lt;</span> <span class="identifier">e</span><span class="special">.</span><span class="identifier">what</span><span class="special">()</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
  <span class="special">}</span>

  <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<h5>
<a name="boost_asio.tutorial.tutdaytime6.h1"></a>
        <span class="phrase"><a name="boost_asio.tutorial.tutdaytime6.the_udp_server_class"></a></span><a class="link" href="tutdaytime6.html#boost_asio.tutorial.tutdaytime6.the_udp_server_class">The udp_server
        class</a>
      </h5>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">udp_server</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
</pre>
<p>
        The constructor initialises a socket to listen on UDP port 13.
      </p>
<pre class="programlisting">  <span class="identifier">udp_server</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_context</span><span class="special">&amp;</span> <span class="identifier">io_context</span><span class="special">)</span>
    <span class="special">:</span> <span class="identifier">socket_</span><span class="special">(</span><span class="identifier">io_context</span><span class="special">,</span> <span class="identifier">udp</span><span class="special">::</span><span class="identifier">endpoint</span><span class="special">(</span><span class="identifier">udp</span><span class="special">::</span><span class="identifier">v4</span><span class="special">(),</span> <span class="number">13</span><span class="special">))</span>
  <span class="special">{</span>
    <span class="identifier">start_receive</span><span class="special">();</span>
  <span class="special">}</span>

<span class="keyword">private</span><span class="special">:</span>
  <span class="keyword">void</span> <span class="identifier">start_receive</span><span class="special">()</span>
  <span class="special">{</span>
</pre>
<p>
        The function <a class="link" href="../reference/basic_datagram_socket/async_receive_from.html" title="basic_datagram_socket::async_receive_from">ip::udp::socket::async_receive_from()</a>
        will cause the application to listen in the background for a new request.
        When such a request is received, the <a class="link" href="../reference/io_context.html" title="io_context">io_context</a>
        object will invoke the <code class="computeroutput"><span class="identifier">handle_receive</span><span class="special">()</span></code> function with two arguments: a value of
        type boost::system::error_code indicating whether the operation succeeded
        or failed, and a <code class="computeroutput"><span class="identifier">size_t</span></code> value
        <code class="computeroutput"><span class="identifier">bytes_transferred</span></code> specifying
        the number of bytes received.
      </p>
<pre class="programlisting">    <span class="identifier">socket_</span><span class="special">.</span><span class="identifier">async_receive_from</span><span class="special">(</span>
        <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span class="special">(</span><span class="identifier">recv_buffer_</span><span class="special">),</span> <span class="identifier">remote_endpoint_</span><span class="special">,</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(&amp;</span><span class="identifier">udp_server</span><span class="special">::</span><span class="identifier">handle_receive</span><span class="special">,</span> <span class="keyword">this</span><span class="special">,</span>
          <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">placeholders</span><span class="special">::</span><span class="identifier">error</span><span class="special">,</span>
          <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">placeholders</span><span class="special">::</span><span class="identifier">bytes_transferred</span><span class="special">));</span>
  <span class="special">}</span>
</pre>
<p>
        The function <code class="computeroutput"><span class="identifier">handle_receive</span><span class="special">()</span></code> will service the client request.
      </p>
<pre class="programlisting">  <span class="keyword">void</span> <span class="identifier">handle_receive</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&amp;</span> <span class="identifier">error</span><span class="special">,</span>
      <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="comment">/*bytes_transferred*/</span><span class="special">)</span>
  <span class="special">{</span>
</pre>
<p>
        The <code class="computeroutput"><span class="identifier">error</span></code> parameter contains
        the result of the asynchronous operation. Since we only provide the 1-byte
        <code class="computeroutput"><span class="identifier">recv_buffer_</span></code> to contain the
        client's request, the <a class="link" href="../reference/io_context.html" title="io_context">io_context</a>
        object would return an error if the client sent anything larger. We can ignore
        such an error if it comes up.
      </p>
<pre class="programlisting">    <span class="keyword">if</span> <span class="special">(!</span><span class="identifier">error</span><span class="special">)</span>
    <span class="special">{</span>
</pre>
<p>
        Determine what we are going to send.
      </p>
<pre class="programlisting">      <span class="identifier">std</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&gt;</span> <span class="identifier">message</span><span class="special">(</span>
          <span class="keyword">new</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">(</span><span class="identifier">make_daytime_string</span><span class="special">()));</span>
</pre>
<p>
        We now call <a class="link" href="../reference/basic_datagram_socket/async_send_to.html" title="basic_datagram_socket::async_send_to">ip::udp::socket::async_send_to()</a>
        to serve the data to the client.
      </p>
<pre class="programlisting">      <span class="identifier">socket_</span><span class="special">.</span><span class="identifier">async_send_to</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span class="special">(*</span><span class="identifier">message</span><span class="special">),</span> <span class="identifier">remote_endpoint_</span><span class="special">,</span>
          <span class="identifier">std</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(&amp;</span><span class="identifier">udp_server</span><span class="special">::</span><span class="identifier">handle_send</span><span class="special">,</span> <span class="keyword">this</span><span class="special">,</span> <span class="identifier">message</span><span class="special">,</span>
            <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">placeholders</span><span class="special">::</span><span class="identifier">error</span><span class="special">,</span>
            <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">placeholders</span><span class="special">::</span><span class="identifier">bytes_transferred</span><span class="special">));</span>
</pre>
<p>
        When initiating the asynchronous operation, and if using <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">bind</span></code>,
        you must specify only the arguments that match the handler's parameter list.
        In this program, both of the argument placeholders (boost::asio::placeholders::error
        and boost::asio::placeholders::bytes_transferred) could potentially have
        been removed.
      </p>
<p>
        Start listening for the next client request.
      </p>
<pre class="programlisting">      <span class="identifier">start_receive</span><span class="special">();</span>
</pre>
<p>
        Any further actions for this client request are now the responsibility of
        <code class="computeroutput"><span class="identifier">handle_send</span><span class="special">()</span></code>.
      </p>
<pre class="programlisting">    <span class="special">}</span>
  <span class="special">}</span>
</pre>
<p>
        The function <code class="computeroutput"><span class="identifier">handle_send</span><span class="special">()</span></code> is invoked after the service request has
        been completed.
      </p>
<pre class="programlisting">  <span class="keyword">void</span> <span class="identifier">handle_send</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&gt;</span> <span class="comment">/*message*/</span><span class="special">,</span>
      <span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&amp;</span> <span class="comment">/*error*/</span><span class="special">,</span>
      <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="comment">/*bytes_transferred*/</span><span class="special">)</span>
  <span class="special">{</span>
  <span class="special">}</span>

  <span class="identifier">udp</span><span class="special">::</span><span class="identifier">socket</span> <span class="identifier">socket_</span><span class="special">;</span>
  <span class="identifier">udp</span><span class="special">::</span><span class="identifier">endpoint</span> <span class="identifier">remote_endpoint_</span><span class="special">;</span>
  <span class="identifier">std</span><span class="special">::</span><span class="identifier">array</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">,</span> <span class="number">1</span><span class="special">&gt;</span> <span class="identifier">recv_buffer_</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
        See the <a class="link" href="tutdaytime6/src.html" title="Source listing for Daytime.6">full source listing</a>
      </p>
<p>
        Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a>
      </p>
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        Previous: <a class="link" href="tutdaytime5.html" title="Daytime.5 - A synchronous UDP daytime server">Daytime.5 - A synchronous
        UDP daytime server</a>
      </p>
<p>
        Next: <a class="link" href="tutdaytime7.html" title="Daytime.7 - A combined TCP/UDP asynchronous server">Daytime.7 - A combined
        TCP/UDP asynchronous server</a>
      </p>
</div>
<div class="copyright-footer">Copyright © 2003-2025 Christopher M.
      Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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